1. Field of the Invention
The present invention relates to a process for producing a semiconductor device using a semiconductor film having crystal structure, more specifically a process for producing a semiconductor device, a typical example of which is a thin film transistor (hereinafter referred to as a TFT). The semiconductor device in the present specification and claims means a device which functions by the use of semiconductor property.
2. Related Art
A monolithic liquid crystal display device wherein a crystalline silicon film is used as an active layer of TFTs to realize high electric field-effect mobility has been generally used as a display device. In the monolithic liquid crystal display device, pixel TFTs constituting a pixel section and TFTs of a driving circuit arranged around the pixel section are formed in a single glass substrate.
Elements which decide electrical characteristics of TFTs depend on the quality of a semiconductor film. Particularly electric field-effect mobility depends on the crystallinity of a semiconductor film. Electric field-effect mobility is directly concerned with the responsibility of TFTs and display ability of a liquid crystal display device produced using TFTs in its circuit. Therefore, researches on methods of forming a crystalline semiconductor film having good crystallinity have been continued. For example, the following method is used: a method of forming an amorphous semiconductor film once and subsequently crystallizing the film by irradiation with a laser ray, or a method of heating the amorphous semiconductor film in an electrically heating furnace to crystallize the film.
However, a semiconductor film formed by such a method is composed of many crystal grains, and the crystal orientation thereof extends in arbitrary directions and cannot be controlled. In this case, therefore, carriers move less smoothly than in a semiconductor made of a monocrystal. As a result, low electric field-effect mobility only can be obtained so that the electrical characteristics of TFTs are restricted.
Thus, in JP-A-7-183540, the Applicant discloses a technique wherein a silicon semiconductor film is crystallized by the addition of a metal element such as nickel, the fact that the metal element has effects of functioning as a catalyst to promote crystallization and lowering required temperature, and a technique wherein crystal orientation can be heightened.
However, the above-mentioned crystallizing method using the metal element having catalyst effect (called the catalyst element herein) has the following problems: the metal element remains in the semiconductor film or on the surface thereof so that the electrical characteristics of TFTs are scattered; the off-state current of the TFTs increases; and the electrical characteristics of individual elements are scattered.
Thus, the Applicant discloses a process of adopting a gettering technique using phosphorus to remove a metal element added to an amorphous silicon film in the step of crystallization from a semiconductor film (particularly from its channel-formed region) even at a heating temperature of about 500° C. For example, by adding phosphorus to the source/grain regions of TFTs and then heat-treating the resultant at 450 to 700° C., a metal element added for crystallization can easily be removed from an element-formed region. This technique about gettering of the catalyst element is disclosed in Japanese Patent No. 3032801.
As described above, the Applicant discloses a process of adopting a low-temperature crystallizing process using a catalyst element and subsequently gettering the catalyst element from a semiconductor film. For example, the following methods are considered: a method of forming a gettering site doped with a group XV element having gettering effect, a typical example of which is phosphorus, at a high concentration, performing heat treatment to move a catalyst element to the gettering site, and subsequently removing the gettering site; and a method of performing a heat treatment step for activating phosphorus added to a region which will be a source region or a drain source afterwards and simultaneously gettering a catalyst element in a semiconductor layer into the source region or the drain region. In these gettering steps, the metal element for crystallization, which is introduced into the semiconductor film, can be removed by performing heat treatment at 550° C. for about 4 hours.
However, in order to satisfy an increase in demand of display devices using TFTs as their circuit, it is necessary to establish a production process so that it can improve productivity and reduce production costs. In order to improve the efficiency of production steps, it has become possible to produce plural TFT substrates from a single large-sized glass substrate (for example, six 12.1-inch type TFT substrates from a 550×650 mm substrate), as illustrated in FIG. 16. However, there is still demanded a technique for producing many TFT substrates from a larger-sized glass substrate or for improving the efficiency of production steps (making the time for production steps short).
The concentration of phosphorus added to the semiconductor film to gain gettering effect is 1020/cm3 or more, preferably 1×1021/cm3. The addition of phosphorus having a high concentration by ion implantation or by ion doping, which means a method wherein ions to be implanted are not subjected to mass separation in the present specification, causes a problem that subsequent recrystallization of the semiconductor film becomes difficult. Furthermore, the time required for the doping is also brought into a problem.